1. Field of the Invention
The present invention relates to stretch draw sheet metal presses and dies.
2. Background Art
Sheet metal parts may be initially formed in a draw press that draws a sheet metal blank into the general shape of the part to be produced. Toggle draw presses have a stationary lower die that is engaged by an upper binder ring that encircles an upper draw punch. The upper draw punch draws the sheet metal panel into a cavity defined by the lower die while the binder ring retains the peripheral edges of the sheet metal blank.
Sheet metal drawing processes may also be performed with stretch draw dies that include an upper die that engages a lower binder that is disposed around a lower draw post. The upper die clamps the sheet metal blank on the lower binder ring and then moves the lower binder ring against the force of a spring mechanism such as a set of nitrogen cylinders, an air pad, or a mechanical spring. Nitrogen cylinders may provide up to a hundred tons of pressure in stretch die forming operations. The upper die continues to draw the sheet metal panel by stretching it over the stationary lower draw post. Other versions of the stretch draw die include a double pad stretch draw die and an inverted toggle die. Both the double pad stretch draw die and the inverted toggle die have a lower binder ring that is driven in the course of the press cycle against the spring mechanism that supports the lower binder while the upper die stretches the sheet metal blank over the lower draw post.
Stretch draw dies offer higher productivity and also offer increased capacity to form high strength alloys that are not easily formed in a conventional toggle draw die. High strength sheet metal parts made of such alloys require that the draw die used to form a panel have higher binder tonnage due to the higher yield strength of the material. In addition, panel designs with deeper draw depths require greater forming travel. Increased forming travel adds to the challenges for sheet metal forming processes.
The above operational requirements may result in excessive press impacts and acoustic emissions. Excessive impacts are transmitted through the press structure resulting in frequent press component failure. This problem results from four main components: high stationary equivalent mass (binder tonnage); weak press configuration (point of first contact is earlier in the stroke due to higher binder travel requirements); higher press speeds; and limited time interval requirements for moving the upper die half to accelerate the lower die half immediately after contact. These four components may result in press failure. Press failures result in a subsequent need to repair stamping presses and a substantial loss of production time.
Disadvantages associated with stretch draw, double pad stretch, and inverted toggle dies include high noise levels that are caused in part by engagement by the upper die with the lower binder. When the upper die engages the lower binder, reaction forces are transmitted to the press structure that can lead to fatigue or failure of the press drive.
Applicants' invention is directed to reducing the effects of impulse and momentum that are inherent in stretch draw forming operations. These and other problems are addressed by applicants' invention as summarized below.